The well known push-pull circuit is diversely applied in the field of electrical engineering and finds use, for example, as an amplifier in power conversion equipment such as dc to ac inverters and dc to dc converters.
Recently, these power control circuits have employed semiconductor devices (such as metal oxide semiconductor field effect transistors) as the power switching elements. While these devices offer considerable advantage in many respects, they are sensitive to transient and long-term overcurrent conditions which, if unprotected against, can cause destruction of the semiconductor switching elements and corresponding failure of the circuitry within which they are employed.
There are at least three causes of overcurrent which can damage these semiconductor power switches:
1. Excessive transient in-rush current at the moment power is initially applied, i.e., at turn-on; PA1 2. Near the end of the active or conducting period, the transformer used in circuits of this type may tend toward saturation resulting in an extraordinarily high current; and, PA1 3. The output load may become excessive due, for example, to an overload or a short circuit on the output of the inverter or converter circuitry.
In view of these problems, it is an object of the present invention to provide protection circuitry, for use in push-pull networks, which guards the semiconductor switches and other circuit elements of the push-pull network against the damaging effects of excessive current. This, and still further objects, advantages, and uses of the invention will be apparent from the ensuing description.